1. Field of the Invention
This invention relates to a bandwidth limiting circuit having a variable bandwidth, which is adapted to vary the passband width of the second intermediate frequency (IF) stage of the converter incorporated in the indoor unit of, for example, a satellite-broadcasting receiver.
2. Description of the Related Art
Satellite-broadcasting receivers, such as television receiving-only (TVRO) receivers, are widely used in the United States. The receiver has an antenna, a low-noise blockdown converter, and an indoor unit. When the antenna receives the frequency-modulated (FM) wave of 3.7 to 4.2 GHz transmitted from a communications satellite, the converter converts this wave into a first IF signal of 0.95 to 1.45 GHz. The first IF signal is supplied to the indoor unit.
The indoor unit comprises a mixer circuit, a local oscillator, a bandpass filter (BPF), and an FM demodulator. The mixer circuit converts the first IF signal to a second IF signal of 140 MHz, by using the output of the local oscillator. The bandpass filter limits the bandwidth of the second IF signal to 27 MHz. The FM demodulator demodulates the frequency of the 27 MHz signal, thereby generating a video signal.
The output bandwidth of the satellite used at present is 36 MHz. Therefore, the bandwidth of the BPF should ideally be set to this frequency. However, as has been stated, this bandwidth is actually 27 MHz for the following reason.
The output noise power N.sub.0 of the receiver is: EQU N.sub.0 =K.multidot.T.multidot.B.multidot.F
where,
K: Boltzmann's constant PA1 T: absolute temperature PA1 B: bandwidth of the BPF PA1 F: noise factor of the receiver
When B is too broad, the carrier/noise (C/N) ratio of the receiver is too small. On the other hand, when B is too narrow, the BPF will delete some part of the input signal, inevitably generating a truncation noise. It has been found that the trade-off value for B is about 27 MHz. This is why a BPF having a bandwidth of 27 MHz is used in the indoor unit of the satellite-broadcasting receiver.
Besides, as is well known in the art, the meteorological changes results in changes in the C/N ratio of the FM waves, i.e., the input signals of the TVRO receivers. Some measures must therefore be taken to cope with the changes in the C/N ratio. Further, in addition to satellites which transmit FM waves of 3.7 to 4.2 GHz, communications satellites are also used which transmit FM waves of 12 GHz. To convert 12 GHz FM waves into video signals, the TVRO receiver must have a BPF whose bandwidth is in the vicinity of 24 MHz.
In the United States, the ground microwave telephone channel uses a frequency close to the central frequency of the TVRO receiver, i.e., 140.+-.10 MHz. Hence, the TVRO receiver may interfere with the telephone microwaves in some areas. To prevent such interference, a notch filter of 140.+-.10 MHz, which has a great Q value, is incorporated at the rear stage of the BPF (27 MHz) which is the second IF stage of the TVRO receiver. The use of the notch filter, however, causes a problem. Since the bandpass width of the second IF stage (i.e., the BPF) of the TVRO receiver is fixed at 27 MHz, the TVRO receiver cannot cope with all FM waves which have different C/N ratios because of the difference in their frequencies (4 GHz and 12 GHz) and also because of the meteorological changes influencing them. The notch filter provided in the second IF stage inevitably cuts off the input signal of any channel other than the interfered one is inevitably cut off.
In order to prevent such an unnecessary cut-off of input signal, a new type of a TVRO receiver has been developed, in which the BPF is replaced by two surface acoustic wave (SAW) filters having different bandpass widths, and these SAW filters are alternatively used. Needless to say, this TVRO receiver requires a means for selecting the first SAW filter or the second SAW filter, is thus complex, and therefore expensive.